1. Field of the Invention
The present invention relates to an electrode plate for a secondary battery with a nonaqueous electrolyte, for example, represented by a lithium ion secondary battery and also relates to a process for producing such electrode plate.
2. Description of the Related Art
In recent years, reductions in size and weight of electronic equipments and communication equipments have been rapidly advanced, and therefore, it has also been required to reduce size and weight of a battery used as a driving power source for these equipments. For this request, there has been proposed a secondary battery with a nonaqueous electrolyte represented as a typical example by a lithium ion secondary battery having high energy density and high voltage.
There has also been required to propose an electrode plate, which significantly affects on the performance of the secondary battery, having large area of a thin film to elongate a charge/discharge cycle life and obtain a high energy density.
For examples, Japanese Patent Laid-open Publication Nos. 10456/1988 and 285262/1991 disclose positive electrode plates which are produced by the steps of dispersing or dissolving an active material powder for a positive electrode plate, which is composed of metallic oxides, sulfides, halides and the like, a conductive agent and a binder into a suitable wetting agent (referred to as "solvent" hereinafter) to prepare an active material coating solution in the form of paste, applying this active material coating solution on a surface of a collector as a substrate made of a metallic foil to form a coating layer (active material coating layer). In this process, as the binder, for example, there is used fluororesin such as polyvinylidene fluoride or silicone-acrylic copolymer.
The binder for preparing the active material coating solution for the above-mentioned coating type electrode plate is required to be chemically stable against the nonaqueous electrolyte, insoluble in the electrolyte, and soluble in a certain solvent to be able to be applied to the surface of the substrate made of a metallic foil. Furthermore, it is also required for the active material coating layer (coating layer) obtained by applying the coating solution and drying the same to have a flexibility so that any peeling, chipping and cracks do not occur to the coat film during the assembling process of the battery and also required to be excellent in adhesive property to the collector made of the mettalic foil.
In usual, for the electrode plate, the existence of the coating layer is unfavorable for a certain portion thereof, for example, a portion to which a terminal for introducing an electric current is connected or a portion at which the electrode plate is bent for preparing a battery. For this reason, the electrode plate is usually formed with at least a portion to which the coating solution is not applied, and a pattern of such not-coated (or non-coating) portion is optionally determined in accordance with a battery design. A method of forming such not-coated portion, in the conventional art, includes one method in which patterns of coating portions and non-coating portions are directly formed under a mechanical control of a coater head at a time of coating the electrode coating solution on the collector and another method in which the coated film after the drying is peeled off by a mechanical means to thereby form the not-coated portion.
In the one method mentioned above, however, it is difficult to form patterns at high speed because of mechanical problem in precision and irregularity of the coating layer thickness. Furthermore, in the another method mentioned above, since the peeling process requires much time, good patterning precision is not provided, or a production of powder at an edge of the peeled portion of the coating layer may be caused. Such methods are therefore not practical for the present industrial execution.
In the prior art mentioned above, some problems have been caused. For example, in the prior art, a collector surface is exposed by applying the active material coating solution only on a portion to be coated of the collector through the mechanical control of the coater to directly form the pattern of the non-coated portion, or by applying the active material coating solution on the entire surface of the collector and then dried to partially peel off the active material layer. As shown in FIG. 16, a terminal 105 is thereafter welded to the exposed collector surface. In this method, however, it is impossible to effectively form a fine pattern to the active material layer 103, and as shown in FIG. 15, the exposed area of the collector at a terminal mounting portion 104 is increased. Because of this reason, there was adapted a method in which a battery capacity is reduced to an amount corresponding to the increased exposed area, and the peeling-off of the active material layer results in loss of expensive material, providing a problem. Furthermore, when a collector having a large exposed area for mounting the terminal is rolled up after the production thereof for the storage or transportation, there may cause problems, for example, of irregularly wound-up condition because of difference in thicknesses of the coated portion and the non-coated portion and of winding strength of the electrode plate such that when the electrode plate is wound up with strong tension, wrinkle may be caused at the active material non-coated portion as the terminal mounting portion or an edge portion of the coating layer may be broken and powder may be produced at the edge portion, and on the other hand, when the electrode plate is loosely wound up, the rolled electrode plate may become loose and be deformed during the transportation to protrude a center portion thereof, thus providing a problem in handling thereof.
Theoretically, such problems may be solved by peeling off the active material layer only at an area to be brought into actually contact with the terminal so that substantially all the portion of the exposed collector surface at the terminal mounting portion is covered by the terminal mounted. In the prior art, however, it is considerably difficult to effectively and economically peel off the active material layer in a desired pattern from the collector surface.
Further, an electrode plate for a secondary battery is produced in mass production through a coating process to prepare an electrode plate having a wide width, and thereafter, through pressing, slit-forming, cutting, group winding, etc. processes. A secondary battery is produced by using such electrode plate through various assembling processes. In order to effectively perform these processes with high accuracy, it is available to apply process control marks, cutting marks, position alignment marks and the like mark to the electrode plate and also apply various identification marks or symbols such as manufacture lot numbers, bar-codes and the like for easy identification and manufacture control of the electrode plate. However, the marking of such identification marks with a printing ink or the like increases the manufacturing step, and in addition to this defect, there involves a problem such that the printing ink marking the identification mark is dissolved in an electrolyte in a battery after the assembling thereof, which adversely affects on the performance of the battery. Because of this problem, it is difficult to properly select the printing ink to be used, and accordingly, the application of the identification marks was practically impossible, the process control or management for the battery manufacturing was made complicated and not effective, defective occurred frequently, and manufacturing cost was increased.